Pigments based on magnetite (Fe.sub.3 O.sub.4) having particle sizes in the range of from 0.1 to 2.0 .mu.m have become widely used. As black coloring pigments they serve to color building materials and lacquers. Their magnetic properties are made use of in one-component toners for photocopiers. For obtaining optimum properties, pigments are generally required to have a very narrow particle size distribution. Such pigments have excellent coloring properties and the magnetic properties demanded by manufacturers of toners, such as high saturation and low coercive field strengths.
The preparation of such pigments is therefore an important objective of development. The processes by which such pigments may be prepared have been amply described.
DE-A No. 900 257, for example, describes the so called precipitation process in which iron-II salts are reacted with atmospheric oxygen in the presence of alkalies to produce Fe.sub.3 O.sub.4 pigments.
Another variation is provided by the two stage precipitation process, in which, as indicated in U.S. Pat. No. 2,631,085, an iron-III oxide or iron-III hydroxide is reacted with iron-II salts with the addition of alkalies to yield magnetite pigments.
Fe.sub.3 O.sub.4 pigments with very good coloring properties and the desired magnetic values may be obtained by this process. Such pigments are also distinguished by their narrow particle size distribution. Under suitable oxidation conditions, .gamma.-Fe.sub.2 O.sub.3 pigments, Bertholoid compounds and iron oxide red pigments (.alpha.-Fe.sub.2 O.sub.3) may be obtained from the aforesaid pigments with preservation of their form.
Magnetite crystallizes in a cubical system with a spinel structure of the structural type O.sub.h 7. The formation of the crystal surfaces (100) results in cubical crystals while the (111) crystal surfaces result in octahedral forms such as have hitherto been observed in magnetite pigments. No definite crystal surface develops if the pigment particles are irregularly formed.
In the above mentioned Patent Specifications, the crystal form is described as cubical, octahedral or irregular. By virtue of their geometry, these pigments can obtain a surface/volume ratio of &gt;1.18.
When the pigments are used for coloring lacquers, the specific requirement for binder depends in the last analysis on the surface/volume ratio. Smaller ratios are aimed at in principle.
It is an object of this invention to provide iron oxide pigments which have a lower surface/volume ratio for a given particle size than the pigments known in the art.